The invention relates to a positioning device comprising an object table, a sub-system for processing an object to be placed on the object table, a drive unit for displacing the object table relative to the sub-system, and a measuring system for measuring a position of the object table relative to the sub-system. The drive unit comprising a stationary part which is fastened to a machine frame of the positioning device, while the measuring system comprises a stationary part and a movable part which is fastened to the object table for cooperation with the stationary part of the measuring system.
The invention further relates to a lithographic device comprising a radiation source, a mask table, a projection system having a main axis, a substrate table, a drive unit for displacing the substrate table relative to the projection system in at least one direction perpendicular to the main axis, and a measuring system for measuring a position of the substrate table relative to the projection system. The drive unit includes a stationary part which is fastened to a machine frame of the lithographic device, while the measuring system comprises a stationary part and a movable part which is fastened to the substrate table for cooperation with the stationary part of the measuring system.
EP-B-0 250 031, to which U.S. Pat. No. 4,737,823 corresponds, discloses a lithographic device of the kind mentioned in the opening paragraphs which comprises a positioning device of the kind mentioned in the opening paragraphs. The object table of the known positioning device corresponds to the substrate table of the known lithographic device, while the sub-system of the known positioning device corresponds to a sub-system comprising the projection system, the mask table, and the radiation source of the known lithographic device. The known lithographic device is used in the manufacture of integrated semiconductor circuits by means of an optical lithographic process. The radiation source of the known lithographic device is a light source, while the projection system is an optical lens system by means of which a partial pattern of an integrated semiconductor circuit, which pattern is present on a mask to be placed on the mask table of the lithographic device, is imaged on a reduced scale on a semiconductor substrate to be placed on the substrate table of the lithographic device. Such a semiconductor substrate comprises a large number of fields on which identical semiconductor circuits are to be provided. The individual fields of the semiconductor substrate are consecutively exposed for this purpose, the semiconductor substrate being in a constant position relative to the mask and the projection system during the exposure of an individual field, while between two consecutive exposure steps a next field of the semiconductor substrate is brought into position relative to the focusing system by means of the drive unit of the lithographic device. This process is repeated a number of times, each time with a different mask with a different partial pattern, so that integrated semiconductor circuits of comparatively complicated structures can be manufactured. The structures of such integrated semiconductor circuits have detail dimensions which lie in the sub-micron range. The partial patterns present on the consecutive masks should accordingly be imaged on said fields of the semiconductor substrate with an accuracy relative to one another which lies in the sub-micron range. The semiconductor substrate should accordingly be positioned relative to the mask and the projection system by means of the drive unit with an accuracy also in the sub-micron range. To obtain such a high positioning accuracy, the position of the substrate table relative to the projection system must be measured by the measuring system of the lithographic device with a corresponding accuracy. For this purpose, the measuring system of the known lithographic device comprises a laser interferometer system. The movable part of the laser interferometer system comprises a mirror which is fastened to the substrate table, while the stationary part of the laser interferometer system comprises a laser interferometer which is fastened to a glass plate which is secured to four vertical columns of the machine frame of the known lithographic device.
A disadvantage of the known lithographic device and the known positioning device applied therein is that the accuracy of the measuring system is adversely affected by mechanical vibrations and deformations of the machine frame. Since the stationary part of the drive unit is fastened to the machine frame, such vibrations and deformations particularly arise under the influence of reaction forces exerted on the stationary part of the drive unit, which reaction forces arise from driving forces exerted by the drive unit on the substrate table. As a result of these vibrations and deformations, the position of the stationary part of the measuring system relative to the projection system is not precisely defined. In particular, the vibrations of the machine frame are comparatively strong when the reaction forces have a frequency which is comparable to a natural frequency which is characteristic of a usual machine frame such as used in the known lithographic device and positioning device. Under such circumstances, the reaction forces will cause the machine frame to resonate, whereby comparatively strong mechanical vibrations arise in the machine frame and in the glass plate carrying the stationary part of the measuring system, even if the reaction forces exerted on the stationary part of the drive unit are comparatively small.